1. Field of the Invention
The present invention relates to a parison stabilizing method and to a system therefor for blow-molding machines. In particular, this invention relates to an improvement which requires that a metal mold at the final stage of a pre-blowing treatment is closed continuously, the pressure in an air passage within a cross head is made negative to quickly expel an excess of air from an external parison located on the outside of the metal mold, so as to control an air pressure (amount) within the parison during the pre-blowing treatment, thus stabilizing the shape of the parison.
2. Description of the Related Art
FIG. 1 shows an example of a conventional blow-molding machine.
In FIG. 1, reference numeral 1 represents a cross head which is connected with an extruder 2. Specifically, the cross head 1 is formed with a melted resin flowing passage 3 having a cylindrical shape.
An air passage 4 is formed through the above cross head 1 and is disposed along the axial line thereof. Connected with the air passage 4 is an exhaust pipe (for releasing air) 6 having an exhaust valve 5. The exhaust pipe 6 is connected with a pre-blow pipe 9 which has a pre-blow valve 7 and is connected with an air source 8 (3 to 10 kg/cm2) located in a plant.
Under the cross head 1, there are provided a parison cutting device 10, a freely openable/closable metal mold 11, and a pre-pinch device 12, disposed independently from one another, thereby forming a multi-stage arrangement.
The operation of the hollow formation molding machine is described in the following. First, the melted resin extruded out of the extruder 2 is caused to move through the melted resin flowing passage 3 and is further extruded downwardly from an outlet opening 3a so as to form a cylindrical parison 20. Then, the cylindrical parison 20 is caused to pass through a metal mold 11 and thus arrive at a pre-pinch device 12. In this way, with the lower end of the cylindrical parison 20 being pinched by the pre-pinch device 12, the metal mold 11 is continuously maintained at its half-closed state, while the pre-blow valve 7 is opened and the exhaust valve 5 is closed, so that the air is supplied into the parison 20, thus carrying out a pre-blowing treatment.
Once the pre-blowing treatment is finished, the metal mold 11 is closed completely, and the external parison 20a extending on the outside of the metal mold 11 is cut by a parison cutting device 10. In a subsequent step, an air nozzle (not shown) is forced into the parison 20 so as to perform a blow forming treatment. That is, a pre-blowing treatment is carried out and thus the parison is pre-expanded. Then, under the conditions, the metal mold 11 is closed completely so as to carry out a predetermined main blowing treatment.
However, since the conventional blow-molding machine is constructed in the above-described manner, there have been the following problems.
That is, at the time the above pre-blowing treatment is being carried out, there will be an excessive amount of air due to the relationship between an introduced air amount and the volume of a cavity formed within the metal mold 11. In fact, such an excess of air will cause the external parison 20a to expand. Furthermore, since it is necessary to perform a puncture, such a puncture will cause unstable molding conditions when forming the parison 20 in the metal mold 11.
At this time, the exhaust valve 5 is opened so that the air is allowed to escape to the outside. Alternatively, the parison is made to expand actively so as to effect the same puncture. In the former case, if the inner diameter of the air passage 4 within the cross head 1 is increased, the air release can be performed smoothly. However, since the outer diameter of the cross head 1 will also have to become large, it is difficult to produce a final product which will satisfy the needs of users. Moreover, there has been a problem in that if the external parison receives a puncture, the parison will be undesirably wound around an take-out device used in a subsequent step.
As a result, in each of the above-mentioned cases, a considerable amount of time is needed to determine various molding conditions until the molding conditions have become stabilized.
The present invention has been accomplished in order to solve the above problems, and it is an object thereof to provide a parison stabilizing method and a system therefor for use in a hollow formation molding machine, which method requires that when a metal mold at the final stage of a pre-blowing treatment is being closed continuously, the pressure in an air passage within a cross head is made negative to quickly expel an excess of air from an external parison located on the outside of the metal mold, so as to control an air pressure (amount) within the parison during the pre-blowing treatment, thus stabilizing the shape of the parison.
The parison stabilizing method according to the present invention is for use in a blow-molding machine which is formed such that after a parison extruded out of a cross head has been introduced into a metal mold, an amount of air is supplied thereto by way of an air passage formed within the cross head to cause the parison to expand, characterized in that the air amount within the parison is controlled by forming a negative pressure within the air passage in the cross head. Furthermore, the parison stabilizing method according to the present invention is characterized in that a forced exhaust pipe is provided outwardly of an exhaust valve of an exhaust pipe which is connected with the air passage, a negative pressure is formed within the air passage by discharging the air through the forced exhaust pipe under a condition in which the exhaust valve is in the open position. Moreover, there is provided a parison stabilizing system which is for use in a blow-molding machine formed such that after a parison extruded out of a cross head has been introduced into a metal mold, an amount of air is supplied thereto by way of an air passage formed within the cross head to cause the parison to expand. The parison stabilizing system comprises an exhaust pipe connected with the air passage and having an exhaust valve; a pre-blow pipe connected between the exhaust valve and the air passage and is further connected to an air source through a pre-blow valve; and a forced exhaust pipe, with one end thereof being connected on the downstream side of the exhaust valve of the exhaust pipe and the other end thereof being connected to the air source. Specifically, a negative pressure is formed within the air passage by at first closing the exhaust valve and the forced exhaust valve, opening the pre-blow valve to perform a pre-blowing treatment in the parison, followed by opening the exhaust valve and the forced exhaust valve, and closing the pre-blow valve so as to cause the air to move to the forced exhaust pipe.